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Aviation History
1909
1909 - 0283.PDF
MAY IS, 1969. . : • Skio-Friction in Air.' Even as late as Langley's experiments, skin-friction in air was regarded as a negligible quantity, but due to the work of Dr. Zahm, who was the first to make any really extensive and reliable experi- ments on skin-friction in air, we now can estimate the magnitude of this quantity. As a result of his research he has given in his paper on atmospheric friction the following equation :— / = 0-00000778 / - <>'<" v 1-ss . .. . (v = ft.-sec), / = 0-0000158 / - o-O7 2,1-86 . . # (t, _ m.p.h.) in which / is the average skin-friction per square foot, and / the length of surface. From this equation the accompanying table of resistances was computed, and is inserted here for the convenience of engineers :— TABLE 2.—Friction per square foot for various speeds and lengths of surface. Average Friction in lbs. per sq. ft. 16-ft. 32 ft. Plane. Plane. tn.p.h. . 5 10 15 20 - 25 • 30• 35 40 50 60 70 80 90 100 0-000303 O"OOII2 OOO237 O-OO4O2 o-00606 0-00850 0-01130 0-0145 0-0219 0-0307 0*0407 00522 0-0650 0-0792 0-000289 o "00105 0-00226 0-00384 o-00579 0-000275 O-OOIOI 0-0021-5 0-00365 0-00551 0-000262 0-000967 0-00205 0-00349 0-00527 o 000250 0-000922 0-00195 0-00332 0-00501 0-00810 O"0108 0-0138 0-0209 0-0293 0-0390 0-0500 0-0621 0-0755 • 0-00772 0-0103 0-0132 0-0199 0-0279 0-0370 0-0474 0-0590 0-0719 0-00736 0-0098 0-0125 0-0190 0-0265 O-O3S3 0-0452 0-0563 0-0685 1 0 00701 I 0-00932 i 0-0125 1 0-0181 i 0-0253 00337 0-0431 0-0536 0-0652 0-000238 0-000878 0-00186 0-00317 0-00478 0-00668 0-00888 0-0114 00172 0-0242 00321 0-0411 0-0511 0-0622 The numbers within the rules represent data coming within the range of observation. These observations show that " the frictional resistance is at least as great for air as water, in proportion to their densities. In other words, it amounts to a decided obstacle in high- speed transportation. In aeronautics it is one of the chief elements of resistance both to hull-shaped bodies and to aero-surfaces gliding at small angles of flight. Relative Dynamic and Buoyant Support. — Peter Cooper- Hewitt has given careful study to the relative behaviour of ships in air and in water. He has made a special study of hydroplanes, and has prepared graphic representations of his results which furnish a valuable forecast of the problem of flight. Without knowing of Helmholz's theorem, Cooper-Hewitt has independently computed curves for ships and hydroplanes from actual data in water, and has employed these curves to solve analo- gous problems in air, using the relative densities of the two media, approximately 800 to I, in order to determine the relative values of support by dynamic reaction and.by displacement for various weights and speeds. An analysis of these curves leads to conclusions of importance, some of which are as follows :— The power consumed in propelling a displacement vessel at any constant speed, supported by air or water, is considered as being two-thirds consumed by skin-resistance, or surface resistance, and one-third consumed by head resistance. Such a vessel will be about ten diameters in length, or should be of such shape that the sum of the power consumed in surface friction and in head resistance will be a minimum (torpedo shape). The power required to overcome friction due to forward move- ment will be about one-eighth as much for a vessel in air as for a vessel of the same weight in water. Leaving other things out of consideration, higher speeds can be obtained in craft of small tonnage by the dynamic reaction type than by the displacement type, for large tonnages the advantages of the displacement of type are manifest. A dirigible balloon carrying the same weight, other things being equal, may be made to travel about twice as fast as a boat for the same power; or to be made to travel at the same speed with the expenditure of about one-eighth of the power. As there are practically always currents in the air reaching at times a velocity of many miles per hour, a dirigible balloon should be constructed with sufficient power to be able to travel at a speed of about 50 m.p.h., in order that it may be available under practical conditions of weather. In other words, it should have substantially as much power as would drive a boat, carrying the same weight, 25 miles an hour, or should have the same ratio of power to size as the " Lusitania." Motors.—It is the general opinion that any one of several types of internal combustion motors at present available is suitable for use with dirigible balloons. With this type, lightness need not be obtained at the sacrifice of efficiency. In the aeroplane, however, lightness per output is a prime consideration, and certainty and reliability of action is demanded, since, if by chance the motor stops, the machine must immediately glide to the earth. A technical discussion of motors would of itself require an extended paper, and may well form the subject of a special communication. Propellers.—The fundamental principles of propellers are the same for air as for water. In both elements, the thrust is directly proportional to the mass of fluid set in motion per second. A great variety of types of propellers have been devised, but, thus far, only the screw-propeller has proved to be of practical value in air. The theory of the screw-propeller in air is substantially the same as for the deeply submerged screw-propeller in water, and therefore does not seem to call for treatment here. There is much need at present for accurate aerodynamic data on the behaviour of screw-propellers in air, and it is hoped that engineers will soon secure such data, and present it in practical form for the use of those interested in airship design. Limitations.—Euclid's familiar "square-cube" theorem con- necting the volumes and surfaces of similar figures, as is well known, operates in favour of increased size of dirigibles, and limits the possible size of heavier-than-air machines in single units and with concentrated load. It appears, however, that both fundamental forms of aerial craft will likely be developed, and that the lighter-than-air type will be the burden-bearing machine of the future, whereas the heavier-than- air type will be limited to comparatively low tonnage, operating at relatively high velocity. The helicopter type of machine may be considered as the limit of the aeroplane, when by constantly increas- ing the speed, the area of the supporting surfaces is continuously reduced until it practically disappears. We may then picture a racing aeroplane propelled by great power, supported largely by the pressure against its body, and with its wings reduced to mere fins which serve to guide and steady its motion. In other words, starting with the aeroplane type, we have the dirigible balloon on the one hand as the tonnage increases, and the helicopter type on the other extreme as the speed increases. Apparently, therefore, no one of these forms will be exclusively used, but each will have its place for the particular work required. : To be continued.) The Vinot Aero-Motor is of the orthodox 4-cyI. type. Its cylinders have a bore and stroke of 103 mm. by 130 mm,, and develop from 36 h.p. at 1,000 r.p.m. to 47 h.p. at 1,600 r.p.m. Without a fly wheel, but with the carburettor and other usual accessories, the weight is 156 kilogs. 28S
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